Transparent, conductive thin films have use as anti-static coatings for television screens, plastics windows, and storage vessels for semiconductor wafers. These thin, transparent, conductive metal oxide coatings have other applications, including antistatic coatings for thermal control dielectric materials on the external surfaces of satellites. Sputtering, chemical vapor deposition and evaporation methods have been used to make these films. These films may also be prepared by dispersing a fine conductive metal oxide powder with desirable optical properties in a polymer binder.
Differential charging can occur on satellite surfaces between dielectric materials of differing properties, and between dielectric and conductive materials. In geosynchronous orbits, these differential voltages can become sufficiently high to cause electrostatic discharges between the materials, and these discharges can both damage electronic circuits and degrade the optical properties of thermal control surfaces. Many thermal control coatings, which are used for passive temperature control of satellite surfaces, contain surface dielectrics such as Kapton or Teflon, and are hence subject to electrostatic discharge problems. Among the approaches used to dissipate surface charging and prevent electrostatic discharges is the application of a thin conductive coating to dielectric surfaces. Such films must be capable of reducing static charge build-up without compromising the thermo-optical properties of the underlying film. Typically, thin, transparent, conductive metal oxide coatings such as indium tin oxide have been used for these antistatic coatings. However, problems with coating adhesion, cracking and peeling of the films during thermal cycling, and penetration by electrons through the thin film to produce trapped charges can reduce the effectiveness of these films as antistatic coatings. These thin films of conductive oxides as spacecraft antistatic coatings have poor adhesion to the Kapton substrates, resulting in cracking and peeling and subsequent loss of conductivity in these films. These problems can be avoided by developing an improved coating technology in which finely dispersed conductive oxide particles are incorporated directly into a suitable polymer matrix to produce flexible, stable antistatic films with suitable thermo-optical properties. Such films could either serve as electrostatic discharge coatings or as a multi-functional thermal control and electrostatic discharge film. In order to successfully develop such a coating, the conductive oxide particles to be incorporated into the polymer matrix must be sufficiently small to remain dispersed in the desired monomer precursor prior to curing of the polymer. Successful dispersion into a polymer matrix requires that the oxide particles be of micron or sub-micron size and not agglomerated.
Indium oxide is an intrinsically semiconducting material that becomes conductive when slightly oxygen deficient, suitable for antistatic coatings. Indium Oxide in a polymer matrix has been used in antistatic coatings. Indium oxide exhibits both the optical and electrical properties necessary for use in these transparent conductive coatings, but is not often used for these applications due to problems with successful dispersion of commercial indium oxide into the polymer binder. Commercial indium oxide powder tends to agglomerate, preventing suitable dispersement in the polymer binder, even when using powerful mechanical mixing techniques such as ball-milling.
The smaller the indium oxide particle size, the better the particles are distributed and suspended in the polymer matrix. Smaller indium oxide particle sizes improve the antistatic properties of the antistatic coatings. Ball milling methods have been used for generating small size indium oxide particles for suspension in a polyimide matrix for antistatic coatings. However, the ball-milling technique is time-consuming and not particularly effective in producing fine, non-agglomerated indium oxide particles. Consequently, there is a need for improved techniques to produce smaller size indium oxide particles suitable for polymer loading.
Ball milling processes have been used on commercial indium tin oxide powder to form particle sizes in the order of 50-500 microns with limited effectiveness in terms of loading and distribution in the soluble polyimide. Sol-gel chemistry has shown that oxides can be prepared in particle sizes from 10 to 100 times smaller than that obtained by ball milling. This particle size distribution would allow better dispersion. Sol gel has not been used to form indium oxide particles. The sol-gel process disadvantageously requires the formation and use of complex intermediate gel phase.
Another technique for generating small size particles is aerosol pyrolysis. Aerosol pyrolysis is a process in which a precursor-containing solution is atomized into droplets. The droplets are transported to a heated region such as in a furnace, for solvent evaporation and precursor decomposition into the desired product. Particles produced by aerosol pyrolysis are typically spherical and uniform in size and composition because the pyrolysis reaction to generate a particle occurs within each self-contained droplet. The size of each generated product particle is determined by the size of the aerosol droplet and by the concentration of precursor within each droplet.
Aerosol pyrolysis has been used to generate small size tin oxide particles using tin-chloride, SnCl.sub.2, oxalic acid, C.sub.2 O.sub.4 H.sub.2, and an ammonium hydroxide, NH.sub.4 OH, pH modifier in water. An aerosol is formed from this solution. The aerosol is then subjected to oxygen and burned to produce the tin oxide, SnO.sub.2, particles. One disadvantage of this aerosol pyrolysis method is the use of a chloride which is corrosive and undesirable in general chemical production.
Indium acetate, as a solid compound, is known to generate, when pyrolyzed, large size oxide particles which are disadvantageous in antistatic polymer coatings. This and other disadvantages are solved or reduced using the present invention.